Change Diffuse shader to emission shader without affecting shader color
This script from http://web.purplefrog.com/~thoth/blender/python-cookbook/change-diffuse-to-emission-node.html should work

import bpy

def replace_with_emission(node, node_tree):
    new_node = node_tree.nodes.new('ShaderNodeEmission')
    connected_sockets_out = []
    sock = node.inputs[0]
    if len(sock.links)&gt;0:
        color_link = sock.links[0].from_socket
    else:
        color_link=None
    defaults_in = sock.default_value[:]

    for sock in node.outputs:
        if len(sock.links)&gt;0:
            connected_sockets_out.append( sock.links[0].to_socket)
        else:
            connected_sockets_out.append(None)

    #print( defaults_in )

    new_node.location = (node.location.x, node.location.y)

    if color_link is not None:
        node_tree.links.new(new_node.inputs[0], color_link)
    new_node.inputs[0].default_value = defaults_in

    if connected_sockets_out[0] is not None:
        node_tree.links.new(connected_sockets_out[0], new_node.outputs[0])


def material_diffuse_to_emission(mat):

    doomed=[]
    for node in mat.node_tree.nodes:
        if node.type=='BSDF_DIFFUSE':
            replace_with_emission(node, mat.node_tree)
            doomed.append(node)

    # wait until we are done iterating and adding before we start wrecking things
    for node in doomed:
        mat.node_tree.nodes.remove(node)


def replace_on_selected_objects():
    mats = set()
    for obj in bpy.context.scene.objects:
        if obj.select:
            for slot in obj.material_slots:
                mats.add(slot.material)

    for mat in mats:
        material_diffuse_to_emission(mat)

def replace_in_all_materials():
    for mat in bpy.data.materials:
        material_diffuse_to_emission(mat)


if True:
    replace_on_selected_objects()
else:
    replace_in_all_materials()


Just paste a copy into a blender Text Editor, select the objects you want adjusted, and click the Run Script button.  If you want to rework all the materials, just change the tail end of the script to run the replace_in_all_materials() function.